The present invention concerns a process for reducing dissolution during subsequent cleaning of a coating having chromic acid or chromate as the principal constituent which coating has been applied to metal sheet such as steel sheet, aluminum sheet or plated steel sheet such as zinc plated steel and zinc-alloy plated steel (e.g. Zn-Al, Zn-Ni, Zn-Fe, Zn-Mn etc.).
It has commonly been known and applied to practical use in each sector concerned that formation, with subsequent drying, of a chromate coating by applying an aqueous solution having chromic acid or chromate as a principal constituent to steel sheet, aluminum sheet and plated steel sheet can provide them with improved corrosion resistance under unpainted as well as under painted conditions. Examples are seen in the products available on the market under the names of Zincrometal and DuraSteel, whose processes consist of, for the former, steel sheet - application of aqueous chromate solution - zinc-rich paint, and for the latter, plated steel sheet - application of aqueous chromate solution - coating of organic/inorganic composite resin.
Generally speaking, dry-in-place type chromate treatment of steel sheet, aluminum sheet or plated steel sheet takes a process [coiled stock - application of aqueous chromate solution - roll squeezing - force drying (sheet temperature: 50.degree.-100.degree. C.) - coiling].
The chromate coating formed on metal sheet surface by such process weighs 5-200 mg/m.sup.2 as the amount of chromium in the coating.
An additional step is also available in dry-in-place type chromate treatment, wherein after the application of the aqueous chromate solution, coating the work with aqueous organic resin solution is conducted to form a coating of 0.1-3 g/m.sup.2. In this case, drying is done at a sheet temperature between 50.degree.-100.degree. C. In case it exceeds 80.degree. C., water cooling is done in the next stage.
Steel sheet chromate-coated as above mentioned is typically subjected to a forming process at user's factory and then to cleaning and painting.
Problems arise from the cleaning stage of the formed work in that part of the chromate coating dissolves out in the cleaning solution, which leads to degradation of the performance of the coating; the dissolved out substance mainly comprises hexavalent chromium which is undesirable also because it contaminates the cleaning solution and poses pollution problems. In order to prevent such dissolution, trivalent chromium or resin have been included in the chromating solution, conducting organic resin or organic/inorganic composite resin coating have been applied after the chromate solution application, or the work has been subjected to high temperature baking after chromate solution application. However, the method of mixing trivalent chromium and resin in the aqueous chromate solution per se is incapable of sufficiently preventing the chromate coating from dissolution, and even organic resin or organic/inorganic composite resin coating applied thereafter cannot afford complete counter-measure. Baking at high temperature after chromate coating also is not preferable from the aspects of energy consumption as well as from the space requirements for erecting a baking oven.
The present invention aims at preventing dissolution of chromate coating during the cleaning stage, thereby keeping the chromate coating at the same level of performance as it has as initially applied as well as for the purpose of preventing hexavalent chromium from dissolving out into the cleaning solution.